This invention relates to an infrared radiation detector comprising a pyroelectric detector element in a hermetically sealed housing.
Infrared radiation detectors comprising pyroelectric detector elements are often referred to briefly as pyroelectric detectors. Pyroelectric detectors are used for a variety of purposes, for example in remote switching systems, in intruder alarms, and in movement sensors generally. Such sensors rely on the fact that a human being naturally provides a moving source of infrared radiation as he walks about or even as he moves only part of his body, for example by waving his hand. The radiation which he emits is converted by the pyroelectric detector into an electric signal which can be used, for example, to actuate an alarm or to switch lights on or off.
The pyroelectric detector element is generally mounted in a hermetically sealed housing to shield it from environmental influences. The housing comprises a canister with an aperture and a window fixed over the aperture. The window is made of a material, such as silicon, which is transparent to infrared radiation. The radiation to be detected may be focussed onto the detector element by, for example, a concave mirror or a convex lens external to the housing. The external mirror or lens is relatively large and, as a distinct entity, has to be aligned and maintained in alignment with respect to the pyroelectric detector.